Television transmitting apparatus



Oct. 31, 1939. MlLLER 2,177,736

TELEVISION TRANSMITTING APPARATUS Filed Oct. 14, 1936 15 mmmm INVENTOR HAROLD MILLER ATTORNEY Patented oct. 31, 1939 TELEVISION TRAN SMITTING APPARATUS Harold Miller, South Ealing, England, assignor to Electric & Musical Industries Limited, Hayes,

Middlesex, England Application October 14, 1936, Serial No. 105,480 In Great Britain October 29, 1935 4 Claims. (Cl. 250-153) My invention relates to cathode ray tube television transmitting apparatus and in particular to tubes of the type which employ light-sensitive electrodes.

In my copending U. S. application, Serial No. 101,213, filed September 17, 1936, there is disclosed a form of cathode ray transmitter tube in which an optical image of an object of which a picture is to be transmitted is formed upon a target electrode including a conductive signal plate having on its front surface a coating of photo-electrically conductive material, the front or illuminated surface being scanned by a cathode ray beam to produce currents which flow through an output resistor connected between the conductive signal plate and ground to generate the picture signals.

The photo-electric conductive effect is exhibited by water-free crystalline materials, either in the form of single crystals, or of compressed polycrystalline powders, having high refractive index, greater than 2, and include (a) Sulphides of many heavy metals such as lead, thallium, mercury, cadmium, silver, zinc, copper and antimony.

(b) Oxides of the above metals.

() Halogen compounds of the first five above-mentioned, and

(12) Some crystalline modifications of certain pure elements such as selenium, sulphur, phosphorus and iodine.

metals the usual way, is exposed to the image to be transmitted and to the scanning cathode ray beam.

According to the present invention, therefore, a sensitive target electrode for a cathode ray television transmitting tube comprises a conductive signal plate bearing on its surface a layer of a photo-electrically conductive material, such for example, as zinc selenide, the layer being coated with a substance which is a better emitter of secondary electrons than the photo-conductive material. A suitable substance is the borate of alkali metal such as lithium, for example,

I have chosen the terms photoconductive effect and photoconductive layer to distinguish my structure from structures which are electron emissive or photo-electrically active when subjected to light.

Other materials may be employed for coating the photo-electric conductive layer as will hereinafter be explained.

In order that the invention may be clearly understood and readily carried into effect, the

:same will be now more fully described with reference to the accompanying diagrammatic drawing in which Fig. 1 schematically illustrates. a cathode ray television transmitting tube embodying my invention, and Figure 2 shows on a greatly enlarged scale a portion of the electrode structure shown in Figure l with parts broken away to show more clearly the film and the coating.

In the drawing, the reference numeral l indicates a target electrode comprising as shown in Figure 2 an electrically conductive signal plate I5 coated on one side with a layer it of photoelectrically conductive material. This target electrode is so mounted in an evacuated envelope 2 that an optical image may be projected on its coated surface by a lens system l2 and also so mounted that the layer I6 may be scanned by an electron beam. Adjacent the target electrode I, is an electrode 3 which serves to collect secondary electrons liberated from the layer 6 when the target electrode is bombarded with a cathode .ray beam which emanates from a cathode 5, is

focused by one or more focusing electrodes (not shown), and is deflected by means of an electromagnetic field generated by coils 6 or by an electrostatic field generatedin known'manner. The conductive signal plate I5 is connected through a resistance 7 to the negative terminal of a battery 8, picture signals being applied through a condenser 9 to an amplifying valve It associated with a leak resistance II.

In accordance with the present invention the layer 16, for the purpose of improving its sensitivity is coated with a thin film ll of a material which is itself a better emitter of secondary electrons than the layer [6 of photo-conductive material. A suitable substance is the borate of an alkali metal such as lithium, for example.

It may be found desirable in some cases that the said substance should not be toany great extent photo-electrically active and lithium b orate is amongst materials which have low photo-sensitivity.

In other cases the alkali metals themselves may be used, and in particular metallic caesium.

Substances such as lithium borate may be applied to the surface of the layer of photo-electrically conductive material by spraying an aqueous solution of the substance. Substancessuch as metallic caesium may be applied in the same way as these substances are applied to photoelectric cathodes. This may be done for example by placing within a side tube, connected to an evacuated envelope containing the screen coated with photo-electrically sensitive material, substances such as caesium chloride and calcium, which evolve metallic caesium vapor on heating. The caesium vapor is caused to condense upon the surface of the screen in a known manner.

The optimum thickness of the film ll of the substance which is the better emitter of secondary electrons is of the order of 10" cms., that is to say several times thinner than the base layer of photo-conductive material, which if the substance is lithium borate may be applied by spraying with a 3% aqueous solution, the period of spraying being relatively short, and if metallic caesium, it may be applied by vaporization. The thickness of the film of caesium should preferably not exceed the above-mentioned amount since the sensitivity decreases with increase of the film thickness above the amount aforesaid.

In operation of the tube shown in the drawing the plate [5 may be maintained at a negative potential of about 50 to volts for example relative to the collector electrode 3. Alternatively, the plate I5 is maintained at substantially the same potential as the collector electrode 3 and in such a case may lag in response of the screen to varying illumination as it is reduced substantially to zero. The effect of applying the additional film in accordance with the invention is to increase the sensitivity for a given beam current by a factor of approximately 2:1, which figure may even be exceeded when caesium is employed.

(1) An optical image of the object to be transmitted is projected through the thin film I! of material, such as lithium borate, which, as previously noted, is of the order of 10- cms. thick, and onto the interface of the film IT and the layer I6. The light passes into the zinc selenide layer l6 which is so thin that the optical image is substantially in focus throughout the thickness of the layer. The resistance from back to front of a section of the layer depends on the intensity of the light falling on that section and potential differences are set up from back to front across different sections of the layer depending on the intensity of the light falling on them.- The cathode ray beam is made to scan the target in a manner well known in the art, which scanning produces currents which flow through the output resistor connected between the conductive signal plate and ground to generate picture signals which correspond in intensity to the degree of light or shade of the optical image.

(2) By adjusting the potential of the battery 8 as stated above, it is found that the delay or lag in electrical response to varying degrees of light and shade of the optical image projected on the target is substantially reduced consistent with satisfactory sensitivity. It is desirable to make a compromise between the factors which govern the lag and sensitivity. In some cases, however, the lag can be entirely eliminated by maintaining the conductive signal plate and the electron collecting electrode at the same potential such, as for example, at earth potential.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed Without departing from the scope of my invention as set forth in the appended claims.

I claim:

1. A target electrode for a cathode ray television transmitting tube comprising a conductive signal plate having on its surface a layer of photoelectrically conductive material which is nonphoto-emissive and a coating on said layer of a borate of an alkali metal.

2. A target electrode for a cathode ray television transmitting tube comprising a conductive signal plate having on its surface a layer of photoelectrically conductive material which is nonphoto-emissive and a film of lithium borate on said layer.

3. A target electrode for a cathode ray television transmitting tube comprising a conductive signal plate having on its surface a layer of waterfree crystalline material which is nonphoto-emissive and has a refractive index greater than 2 and a coating of lithium borate on said layer.

4. A target electrode for a cathode ray television transmitting tube comprising a conductive signal plate having on its surface a nonphotoemissive layer of photoconductive material comprising copper oxide and a coating on said layer of a substance which is a better emitter of secondary electrons than the photoconductive mate a HAROLD MILLER. 

